Hermetically sealed resistor



April 3, 1951 F. o. MITCHELL ETAL 2,547,405

HERMETICALLY SEALED RESISTOR Filed June '7, 1945 INVENTORS f/ffDf/f/f' 0. MIT l/fl Jr 00/1/1410/10' l [0 4/7 Patented Apr. 3, 1951 UNITED STATES PATENT OFFICE Frederica Mitc ell,- Collingdale, anaiit iaia' Hall'oran, Philadelphia, Pa a sjs'ignors' to Shallcross Mfg. Company, Colli'ngdale', Pa.

Application June 7,:1945,=Serial No. 598,070

16 claims. 1

The present invention relates to method and means for hermetically sealingelectrical units such as resistors, condensers, and coils; The principles of the invention will be described in connection with resistors-by Way of example.

7 In electrical units, such as resistors, intended for installation and main electrical apparatus, radio; electrical measuring instruments, testing equipment, and the like; a high degree of; precisionand: accuracy is required to bemaintainedundermany and-varied conditions of service, and the electrical characteristics-thereof are readily disturbed byconditions' of high humidity, moisture, extremes of temperature, and vibrations; Various attempts have been made to protect resistors from leakage losses, but sucharrangements ashave-come-to our knowledge are fragile and bulky, or require undesirable manufacturing processes, or special mountings; and do notprotect; the resistors otherwise;

It is' oneobject of our invention to provide a hermetically sealed resistor, or other electrical unit, the functioning of which is unaffected by humidity and moisture, wide temperature variations,--vibration s-and rough handling; V,

Other objects of our invention are to provide; a method of manu'facture of hermetically sealed resistors, and the like, which is capable" of use inmassprcduction methods, and may" be used with wire woundresistors of either the multiple layer or thesingle' layer type.

Further objects ofour invention are toprovide unitary herme'zically'sealed resistors, and the like, inwhich the connecting wire portion does'not extend outside the" sealand no external circuit connection extends inwardly beyond theseal; and wherein means may be providedto prevent the deleterious effects of the products of decomposition Within thehermetically sealed unit.

Still' further objects ofour invention are to provide hermetically sealed units which may be nicuntedin a -very simple manner 'in'fact in the same manner as 'unsealed resistors and the like now beingmarketed, and likewise. one in which normal or usual terminal connectionarrangements may be used.

Otherobjects andfeatures of our inventionby which we have produced a hermetically sealed electrical unit that is efficient and durable will be understood from this'specification takenin conjunction with" the accompanying drawings,- wherein- I Figure 1 is an elevational viewof an assembled, hermetically sealed resistor, equipped with termi- V 2 c nallugs, parts being broken away to facilitate ei tr o t i o. c

Fig. 2 is a'central-sectionalView of the winding bobbin; a h Fig.3 is aperspective view of the winding bobbin} t. Fig. 4- is aperaspective view ofthe covering f i i V, i V Fig. 5 is a fragmentary detail in perspective of the starting; end of; the b9bbin; and H 7 Figs. 6 '7 8 and 9 are fragmentary views of one end of the resiston illustrating structure-and steps sealingthe-resistbr'. U i w In the drawings, the-parts; are shown enlarged om 1 m?and:,.l tlt in lu r i It Wil-lbe understood-that the illustration and description of the principles of this inventionqn e i n it W r W un e is rs s way of; example only the disclosure nevertheless being ofthe best mode of practicing'} the invention" in-- sofar as'the -same is known to us. v In carrying out our' invention as illustratec i; abobbing for the resistance wire has closedends fitting inside a sleeve the internal diameter of; which is substantially the same as theflexternal am e t f ai ediacen o t s of the ends of; the sleeve andbobbin are provided with metallized surfaces andin-thes ealingoperations thesem'etalliz'ed: portions-v are joined together; by; means" o-f solder; The construction is-. suchthat no part of the res stance wire: is brought to theoutside of the unit, and'no-partofitheconnecting terminalor mounting; arrangement eXtend sinside the sealed;unita In-thesealingoperations a; partiaL vacuum is created inside the unit and provisions aremadewhereby this vacuum may bepreserved; v Referring to the drawings, in wire woundrre sistors, resistance wireof suitable size for the desired selec-tedvalue is'wound upona-bobbin: H; comprising-acme 2 and enddiscs orflanges-i it? In the: multiple winding; type one or more intermediate discs; or; flanges M; preferably: of less diameter than the ;end discs; are distributedalong the core I-2=- The end discs-are alikesand comprise innerand outersections HA and Iii-B separated by ashallow circumferential groove.

I5.; The;innersections |3A are slightly largeriii-diameter; than the outer sections I3Bfor-pu1 pose s hereinafter described: At the end of the bobbin w-he-reth'e winding: is started; the inner section is provided with a;-rad ial"slot l6 extend? ingto the core-L- and opening; inwardly; At the oppositeorfinish endfthe inner section of' the, end disehas ashort tapered slot I l which" also opens inwardly. The intermediate discs I4, where used, also are provided with a radial slot l8.

Extending outwardly from each end disc is an extension or head [9, of reduced diameter, and joined to the disc section 1313 by the undercut throat 20. The heads I9 and throats 20 are each provided with a radial groove 2|.

The bobbin, except for the protruding heads I9, is, in final assembly, enclosed by the cylindrical shell 26. As will be seen from Fig. 2, the internal diameter of the shell is substantially the same as that of the sections I3Aof the end discs, and the ends of the shell are aligned with the outer surfaces of the sections I313 of the end discs. In the assembled position, therefore, the only points of support between the bobbin and shell are the end disc sections I3A until the bobbin and shell are sealed together, as will be described.

The resistor carries terminal lugs 21 which are fastened at each end of the bobbin. The terminal lugs are alike, each having a pair of legs 28 at one end thereof and arranged to be received in the throat 20, as seen in Fig. 8. The free ends of the legs 28 are turned inwardly into the groove 2|, as indicated at 29, to clamp the lug in place. At the start of the assembly operation the lugs are bent outwardly, as indicated at 3|. (Fig. 8.)

Before starting winding of the resistor, a few turns of one end of the wire are laid in the groove [5, this portion having been stripped of all insulation. The resistance wire is usually extremely small in diameter, and to facilitate the obtaining of a good mechanical and electrical connection, it is usual to wrap a few turns of a heavier wire in the groove so as to overlie the turns of resistance wire therein. This arrangement is seen in Fig. 5, the resistance wire being indicated at 32 and the overlying bonding wire at 33. The groove [5 is then substantially filled with solder, but care is taken, according to our construction, to ,leave no protruding ends of the resistance wire or of the heavier bonding wire. The resistance wire 32' is usually enamel covered, and the wire, so covered, is brought from the groove I5, through the radial slot IE to the core l2. Thereafter the wire is wrapped upon the core between the portions I3A and I4, in proper amount according to the articular selected value. The resistance wire is then extended from the first coil through the radial slot I8 in the intermediate disc l4, and a second coil is wound upon the bobbin between the disc I4 and the opposite end portion 13A. Usually, but not necessarily, these windings are in reverse directions so as to reduce or eliminate inductive effects therebetween. I

When the required length of wire has been wound upon the bobbin, the wire is passed through the groove l1 and a few turns thereof are wrapped around the groove l5 in that end disc, but the wire is not sealed therein at this stage. To insure accuracy of the resistor, the same may be properly aged at temperatures high enough to relieve any mechanical and other stresses which might have been set up during the winding process. This aging is usually carried out from 24 to 48 hours at temperatures from 120 C. to 130 C. After the aging, the resistor is calibrated and adjusted, following which a few turns of the resistor wire, stripped of insulation, are wrapped in the groove I5 at the finish end of the bobbin; and bonded in place as heretofore described for the starting end, viz., by wrapping 4 heavier bonding wire thereover and soldering, again exercising care that no end of the bonding wire or resistor wire protrudes outside the groove.

The bobbin II and the shell 26 are preferably formed of a suitable ceramic material, such as steatite, and the outer surfaces of the shell and bobbin may be glazed.

The structure and method of hermetic sealing the unit will now be described. As indicated in Figs. 6 to 8, the groove I5, the perimeter and outer 'fiat surface of the end section I313 and the throat 211, including that portion of the groove 2| which extends into the throat 20, are all provided with a metallized coating 35. Any suitable metallized coating may be used, one which we have found satisfactory consisting of an electro-deposited layer of silver over which is plated a layer of copper and over which is an outer layer of tin. Each edge of the cylinder is provided with a similar metallized coating, indicated by the cross hatching in Fig. 6. The metallized coating is continued on the interior surface of the cylinder to form a band 30 at each end of the shell, which bands are at least as wide as the area of the perimeter of the end disc I3B plus the groove I5. When the shell 26 is mounted in place on the bobbin, the coating on the edge portions thereof is flush with the metallized coating on the fiat portion of the adjacent end disc [33, this relationship being suggested in Fig. 2. It will be understood that in all instances the metallized coatings are shown in exaggerated proportions in the drawings, for the sake of clarity.

To accomplish the hermetic sealing, the following procedure has been found effective: the ceramic shell 26 is fitted over the bobbin II as indicated in Fig. 2, and the metallized coatings at one end of the bobbin and shell are soldered together, the solder 35 (Fig. 1) being flowed into the annular joint between the end disc 13B and the shell 25, and the protruding portion I3A forming a barrier to the further passage of the solder into the interior of the unit. The solder also covers all the end of the assembly, except the head [9, and is flowed into the throat 20 and its groove 2 I, and over the now straightened lug 21, as illustrated in Fig. 9.

With one end of the resistor unit thus completely sealed, the unit is heated in an oven for a period of approximately one hour at a temperature of the order of C. to drive moisture and gases from the interior of the unit through the unsealed end of the unit. At the end of this baking or heating step, and while the unit is still hot, the unsealed end is sealed in the same manner as described above for the opposite end, namely, the two metallized coatings being bonded together and the solder flowed over the end flat surfaces and 111g and into the throat 20. The unit is then allowed to air cool in the open room, and when the unit has reached room temperature it is subjected to final test and checking. The vacuum created in these final heating and cooling steps is preserved by the sealing, and the vacuum tends to draw the solder further into the joint between the end disc and the shell at the finish end.

From the foregoing description it will be seen that a rigid, unitary construction is produced and wherein no part of the resistor wire or its connecting lead is brought outside the unit and no part of the external connecting terminal extends inside the unit. Instead, the electrical circuit connection therebetween is made through the sealing material. 7

The finished resistor unit may be mounted by any suitable means, the form described allowing the use of a clamp about the shell; or a central hole 36, extending entirely through the bobbin, may be provided for mounting purposes.

, Certain other optional and alternate matters and arrangements may be noted here. For example, before any winding is put on the bobbin, all the interior surfaces thereof may be coated with a suitable protective lacquer which may or may not be of the thermosetting type. The glazed outer surface of the shell, useful in reduc ing the formation of a leakage path between the ends of the shell and the terminals especially under conditions of high humidity, may be re placed by a coating of a moisture'proof, fungus resisting substance such as a thermo-plastic organic resinous compound containing a fungicide.

apparently is subject to deterioration and decomposition under some conditions of use, notwithstanding the hermetic sealing of the resistor, andthe result is failure of the resistor. It has not been determined whether or not this is the source of gases and volatile decomposition products, but if present, these may be desirably and effectively absorbed by the use of a suitable absorbent material, such as powdered activated charcoal. flhis charcoal may be put inside the resistor in any suitable manner. For example, it may be incorporated in a lacquer vehicle. Strips or Furthermore, the enamel coating on the wire,

, said unit and electrically connected to bands thereof may be painted on the interior 2 of the shell 2'! before assembly thereof, and it is desirable to have the same opposite each section of the resistor as they are defined by the one or more intermediate discs it.

While reference has been made above to the use of a bobbin and shell of steatite and the particular metallized coating bonded thereto, it will be understood that other materials may be used throughout, in which event the times and conditions of the steps in the method of hermetically sealing will be correspondingly varied, as will be understood. Also, differences in size or" the unit will necessitate variations in the times of treatment. The above description and illustration have been made in connection with one type, of wire wound resistor but the principles set forth are applicable to the other types of wire wound resistors, as will be apparent to those acquainted with this industry. It is also obvious that with the construction described above other methods and means may be used for sealing the unit.

We claim:

l. A hermetically sealed resistor unit comprising a ceramic bobbin having a pair of end discs, metallized, coating bonded to the ceramic ma terial about at least a portion of the perimeter of each end disc, resistor wire carried by saidbc-bbin, the ends of said wire terminating at and being secured to saidv metallized coatings respectively,,a ceramic shell in which the bobbin. is enclosed, the shell having bands of metallized coat ing in position to and of a size tocooperatewith the. coated portions of said end discs, and the metallized coatings of the end discs and shell ,being joined by solder so as to entirely seal the respective joints between the shell and the end discs of the bobbin and also establish eiectrical connections with the ends of said resistor wire.

bobbin, the ends of said who terminating at and being secured to said metallized coatings respectively, a ceramic shell in which the bobbin is enclosed, the shell having bands of metallized coating in position to and of a size to cooperate with the coated portions of said end discs, the metal lized coatings of the end discs and shell being joined by solder so as to entirely seal the respecof the bobbin and also establish electrical con-- nections with the ends of said resistor wire, thesolder extending from said jointsexternally of the sealed unit, and terminal lugs mounted on said solder externally of the sealed unit.

3. A hermetically sealed resistor unit compris ing a ceramic bobbin having a pair of end discs-I metallized coating bonded to the ceramic material about at least a portion of the perimeter of each end disc and externally thereof, resistor wire carried by said bobbin, electrical connectors from said wire and terminating at and being secured to said metallized coatings respectively, a ceramic shell in which the bobbin is enclosed, the shell having bands of metallized coatingin po-' sition to and of a-size to cooperate with the coated portions of said end discs, and a portion of the shell coating extending externally of the assembled bobbin and shell, the metallized coatings of the end discs and shell being sealed together so as to entirely close the respective joints between the shell and the end discs of the bobbin and with said connectors inside the sealed portion sothat the metallized coatings form part of the electrical 2 circuit, and terminal lugs mounted on said unit and electrically connected to the metallized coatings externally of the sealed unit.

4. A hermetically sealed resistor unit comprising a ceramic bobbin having a pair of end discs, metallized coating bonded to the ceramic material about at least a portion of the perimeter of each end disc, the portions of the bobbin between said end discs having a protective coating, resistor wire carried by said bobbin, electrical connectors from said Wire and terminating at and being secured to said metallized coatings respectively, a ceramic shell in which thebobbin is enclosed, the shell having bands of metallized coating in position to and of a size to cooperate with C the coated portions of said end discs, the metallized coatings of the end discs and shell being sealed together so as to entirely close the respective joints between the shell and the end discs of the bobbin and also establish electrical connections with said connectors, terminal lugs mounted on said unit and electrically connected to the sealed metallized coatings externally of the sealed unit, and a protective coating over the exterior of the shell between the ends thereof.

5. A hermetically sealed resistor unit comprising a member carrying the resistance element and having a pair of'end discs, metallized coating bonded to and about at least a portion of the perimeter of each end disc, electrical connectors rom said element and terminating at said metal-- lized coatings respectively, a shell in which the member and end discs are enclosed, the shell having bands of metallized coating in position to and of a size to cooperate with the coated portions of said end discs the metallized coatings of the end discs and shell being sealed together so as to entirely close the respective joints between the shell and the enddiscs and also establish electrical connections with said connectors, terminal lugs mounted on said unit and electrically connected to the sealed metallized coatings externally of the sealed unit, and protective coatings applied over the interior and exterior portions of the unit between the ends thereof.

6. A hermetically sealed resistor unit comprise ing a member carrying the resistance element and having a pair of end discs, metallized coating bonded to and about at least a portion of the perimeter of each end disc, electrical connectors from said element and terminating at said metallized coatings respectively, a shell in which the member and end discs are enclosed, the shell having bands of metallized coating in position to and of a size to cooperate with the coated portions of said end discs, the metallized coating of the end discs and shell being sealed together so as to en: tirely close the respective joints between the shell and the end discs and also establish electrical connections with said connectors.

7. A hermetically sealed electrical unit comprising a mounting member for the unit and having a pair of end discs, metallized coating bonded about at least a portion of the perimeter of each end disc, electrical connectors extending from the unit and terminating at said metallized coatings respectively, a shell in which the mounting member and end discs are enclosed, the shell having on the interior thereof bands of metallized coating in position to and of a size to cooperate with the coated portions of said end discs, the metallized coatings of the end discs and shell being sealed together so as to entirely close the respective joints between the assembled shell and end discs and also establish electrical connections with said connectors.

8. A hermetically sealed electrical unit comprising a mounting member for the unit and having a pair of end discs, metallized coating bonded to and about at least a portion of the perimeter of each end disc, a shell in which the mounting member and end discs are enclosed, the shell having on the interior thereof bands of metallized coating in position to and of a size to cooperate with the coated portions of said end discs,the metallized coatings of the end discs and shell being sealed together so as to entirely close the joints between the shell and the end discs.

9. The method of hermetically sealing an electrical resistor unit comprising a mounting member having end discs provided with metallized coatings bonded thereon and a cooperating enclosing shell with complementary metallized coatings thereon, which consists in first soldering together the metallized coatings of one of the end discs and shell, then baking the unit to expel the moisture and other vapors through the unsoldered end and to dry the air on the interior of the shell, and before the unit cools soldering together the complementary metallized coatings at the opposite end of the unit so as to completely close the same.

10. In a hermetically sealed resistor, a bobbin having a pair of end discs respectively provided with circumferential grooves, heads of reduced diameter extending outwardly from said end discs .and joined thereto by means of an undercut throat, a metallized coating over each end disc of the bobbin, said coating being applied in the circumferential grooves and throats as well as over at least a portion of the perimeter of the end discs and the outer flat faces thereof, resistance wire wound upon said bobbin and having the ends thereof received in and terminating in said circumferential grooves respectively and soldered to themetallized coating therein, a cylindrical shell enclosing said bobbin and having bands of metallized coating on the edge portions thereof and internally of the shell,the shell and bobbin being of a size that when assembled together the metallized coatings of the end discs and shell are adjacent each other, terminal lugs secured in each of the respective throats, the re-'. spective sets of metallized coatings being soldered together in a manner to completely close the joints therebetween, the layer of solder extending across the end discs and into the throats so as to form continuous mechanical and electrical connections between the terminal lugs and the resistance wire through the metallized coatmg. I

11. In an hermetically sealed resistor, a bobbin having a pair of end discs and one or more intermediate discs of less diameter, a head also of reduced diameter extending outwardly from each of said end discs and joined thereto by means of an undercut throat, a plurality of windings of resistance wire upon said bobbin, a shell enclosing said bobbin and end discs, the shell and bobbin being of a size respectively that only a narrow gap exists between the peripheries of the end discs and the shell, and means for sealing to'-'- gether the end discs and shell to close the space within said shell, with the heads and throats outside the sealed space.

12. An hermetically sealed electrical unit comprising a mounting member for the unit and having a pair of end discs, metallized coating bonded about at least a portion of the perimeter of each end disc, electrical connectors extending from the unit and terminating at one end at said metallized coatings respectively, a shell in which the mounting member and end discs are enclosed, the shell having on the interior thereof bands of metallized coating in position to and of a size to cooperate with the coated portions of said end discs, the metallized coatings of the end discs and shell being sealed together so as to entirely close the respective joints between the assembled shell and end discs and in electrical circuit connection with said connectors, and an absorbent material for gases within the enclosed space.

13. An hermetically sealed resistor unit comprising a hollow bobbin having a pair of end discs respectively provided with a peripheral groove, resistor wire mounted upon said bobbin and having end lengths thereof located in said grooves, electrical connector elements engaging said end lengths in said grooves respectively and extending outwardly in said grooves, a metallized coating on each of said end discs and engaging the respective electrical connector elements, a shell enclosing the bobbin and provided with metallized coating in position to cooperate with the coated portions of said end discs, and means for sealing together the metallized coatings of the end discs and shell so as to completely enclose the bobbin and the grooved portions of its end 'discs in the shell, the sealed area being defined by the shell and the end discs.

14. An hermetically sealed resistor unit comprising a hollow bobbin having a pair of end discs respectively provided with a peripheral groove and an external head spaced from each disc, resistor wire mounted upon said bobbin and having end lengths thereof located in aid grooves,

electrical connector elements engaging said end lengths in said grooves and extending outwardly in said grooves, a metallized coating on each of said end discs electrically engaging the respective connector elements and extending into said external spaces, a shell enclosing the bobbin and provided with metallized coatings in position to cooperate with the coated portions of said end discs, means for sealing together the metallized coatings of the end discs and shell so as to completely enclose the bobbin and the grooved portions of said end discs in the shell, and electrical terminal connections mounted in said spaces and connected to the projecting portions of the metallized coatings on said end discs.

15. A hermetically sealed electrical unit comprising a pair of discs, said unit being supported by said discs, metallized coating bonded about at least a portion of the perimeter of each disc, a shell in which said unit and discs are enclosed,

said shell having on the interior thereof bands of metallized coating in position to and of a size to cooperate with the coated portions of said discs, the metallized coatings of the discs and shell being sealed together so as to close the joints between the shell and the discs, and electrical circuit connections from the enclosed unit and extending externally of said discs.

10 16. A hermetically sealed electrical unit comprising a pair of discs, said unit being located therebetween, metallized coating about at least a portion of the perimeter of each disc, a shell in which said unit and discs are enclosed, said shell having bands of metallized coating in position to and of a size to cooperate with the coated portions of said discs, the metallized coatings of the discs and shell being sealed together so as to close the joints between the shell and discs, and electrical circuit connections from the enclosed unit and extending externally of said discs.

FREDERIC D. MITCHELL.

DONALD M. OHALLORAN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,842,191 Oppenheim Jan. 19, 1932 2,003,175 Daly May 28, 1935 2,215,587 Kerschbaum Sept. 24, 1940 2,265,821 Siegel Dec. 9, 1941 

